Various adhesive compositions may be used in bonding a broad variety of adherends, including thermoset plastics, thermoplastics and metals. Thermoset plastics may include plastic compositions which may or may not be reinforced with high strength fibers. Examples include fiber reinforced phenolic, polyester, vinyl ester, polyurethanes and epoxy resins. These types of thermoset plastics adherends may be fabricated with various methods including open molding or hand lay-up, sheet molding, resin transfer molding, pultruded molding, centrifugal cast molding, infusion molding and other types of molding processes can be used as well. A finished part can be molded with an in-mold coating, typically to the front surface or cosmetic side of molded part (“A” side surface), whereas the back side of molded part (or “B” side) can be open to cure in the air (open molded) or covered with another mold to cure without presence of air (closed molded) and can either be gel-coated or non-gel-coated. These molding methods can provide vastly different surfaces or adherends.
In addition, mold release compounds can be used to help reduce surface tension of parts to allow for easy release of the finished parts from a mold and can be externally applied to a surface of the mold before fabrication, added to the resin system or internally applied and are forced out while curing. As these mold release compounds are designed to alter surface chemistry or even reduce surface tension of the adherend, they affect the adhesion properties and force additional preparation of substrates (e.g. wiping with alcohol; sanding to remove this surface). Examples of mold release compounds include simple wax types that are applied frequently and come off after a few molding cycles (e.g. bee wax or carnauba wax to polymeric or synthetic resins that bond to mold surface and reduce surface tension). Internal release compounds are forced out while thermoset resin is curing to the surface or exterior surface of part and create lower surface tension or energy. These can be simple (e.g. vegetable oil), complex (e.g. zinc stearates, calcium stearates) or polymeric systems.
Mold release compounds can be used in thermoplastic molding as well. However, thermoplastic adherends present different bonding parameters according to the composition of the thermoplastic resin and surface tension or surface energy of a molded part.
Metals and metal alloys (e.g. carbon steel, stainless steel and aluminum) present a different set of bonding parameters than either thermoset plastics or thermoplastics. They can be coated with oil/grease, protected with a polymer film or a coating (e.g. epoxy or polyester powder coating), or dipped or electroplated with a metallic film (e.g. zinc galvanized, zinc chromate). Without a coating, the metal surface can oxidize and cause under-film corrosion between metal layer and adhesive bond, causing premature bond failure.
The various adherends, whether thermoset, thermoplastic or metal have widely different surface energies and bonding characteristics which makes bonding multiple adherend types using a single adhesive composition difficult. As such, there is a need for an adhesive system formulated with the appropriate elastomeric polymer, core/shell polymer, acrylate or methacrylate based resins, monomers, and curing additives for providing adequate bonding across various adherends with a single adhesive composition platform.